It takes 21 minutes for 10 people to paint 10 walls. How many minutes does it take 13 people to paint 13 walls?
Imagine that each person is assigned one wall, and all 10 people begin painting at the same time. Since everyone will finish painting their assigned wall after 21 minutes, it takes one person 21 minutes to paint one wall. If we have 13 people and 13 walls, we can again assign one wall to each person. Everyone will take 21 minutes to paint their assigned wall. In other words, it takes 21 minutes for 13 people to paint 13 walls.